Mild copper-mediated direct oxidative cross-coupling of 1,3,4-oxadiazoles with polyfluoroarenes by using dioxygen as oxidant

Article abstract of DOI:10.1002/chem.201204502

CH₃CN and O₂ do the trick! A copper-mediated direct oxidative cross-coupling of 2-(het)aryl-1,3,4-oxadiazoles with polyhaloarenes under mild reaction conditions has been developed (see scheme). The process provides a concise access to biaryl structures containing polyhaloarenes, which are of interest in the fields of pharmaceuticals and functional materials. Acetonitrile and oxygen play crucial roles. Copyright

Full text of DOI:10.1002/chem.201204502

DOI: 10.1002/chem.201204502
Mild Copper-Mediated Direct Oxidative Cross-Coupling of 1,3,4-Oxadiazoles
with Polyfluoroarenes by Using Dioxygen as Oxidant
Liang-Hua Zou, Jakob Mottweiler, Daniel L. Priebbenow, Jun Wang,
Jan Alexander Stubenrauch, and Carsten Bolm*[a]
In recent decades, significant progress has been made in
À
À
transition-metal-catalyzed carbon carbon and carbon
ACHTUNGTRENNUNG
heteroatom bond-forming reactions.[1] Although preactiva-
tion by halogenation and/or metalation is commonly re-
À
quired, recent advances in direct metal-catalyzed C H func-
tionalizations have offered alternative synthetic pathways,
which often appear to be more efficient.[2] Along these lines,
various biaryls have been prepared by heteroarene–hetero-
ACHTUNGTRENNUNG
arene,[3] heteroarene–arene,[4] and directed arene–arene[5]
Scheme 1. Synthetic approaches to 2-aryl-1,3,4-oxadiazoles.
À
coupling reactions involving C C bond formation through
À
dual C H bond cleavage. Unfortunately, many of these pro-
À
cedures require the use of expensive palladium catalysts in
combination with toxic reagents and rather harsh reaction
conditions. Furthermore, in many cases, the use of stoichio-
metric amounts of both a suitable metal salt and an oxidant
is necessary.[6] Improved protocols have recently been devel-
Noting the significant advances made in C H functional-
N
progress in copper-catalyzed oxidative cross-coupling reac-
tions,[7,16,17] we decided to investigate the oxidative C5-aryla-
À
À
tion of 1,3,4-oxadiazoles through Csp2 H/Csp2 H cross-cou-
pling reactions. Herein, we report on copper-mediated reac-
tions between 2-substituted 1,3,4-oxadiazoles and polyfluo-
À
oped and successful copper-catalyzed direct oxidative C H/
[7]
À
C H cross-coupling reactions have been described. How-
À
À
ever, those that involve Csp2 H/Csp2 H coupling reactions
ACHTUNGTRENrNGNU oarenes with dioxygen as oxidant at room temperature
are still rare.[8]
(Scheme 1, route c).
1,3,4-Oxadiazoles are important pharmacophoric core
structures of compounds exhibiting a broad range of bioac-
tivities. For example, various derivatives of this heterocycle
have been found to be fungicidal,[9] antimicrobial, and anti-
bacterial.[10] In medicinal chemistry[11] and applied material
sciences relating to electron-transport devices and organic
light-emitting diodes (OLEDs),[12] polyfluorinated biaryls
are of high importance. In 1967, Vorozhtsov and co-work-
ers[13] reported the synthesis of 2-(pentafluorophenyl)-5-
phenyl-1,3,4-oxadiazole by cyclization of 1-benzoyl-2-penta-
fluorobenzoylhydrazine (Scheme 1, route a), prepared by
treating pentafluorobenzoyl chloride with benzohydrazide.
Recently, a series of 2-aryl-5-phenyl-1,3,4-oxadiazoles were
synthesized by Miura and co-workers by copper-mediated
direct arylation of 1,3,4-oxadiazoles using aryl halides
(Scheme 1, route b).[14]
For the initial screening (Table 1), 2-phenyl-1,3,4-oxadi-
AHCTUNGTREGaNNNU zole (1a) and pentafluorobenzene (2a) were used as start-
ing materials. After significant experimentation it was found
that the desired product 3a could be obtained in 34% yield
when 1a and 2a (0.2 mmol scale) were applied in a 1:5 ratio
in the presence of one equivalent of CuBr, three equivalents
of tBuOLi, one equivalent of 1,10-phenanthroline, and ace-
tonitrile as solvent under an atmosphere of dioxygen
(Table 1, entry 1). Lower yields of 3a were observed when
the reaction was performed in benzonitrile or propionitrile
instead of acetonitrile. No product was obtained in other
solvents (toluene, DMF, DMSO, NMP, and THF). For fur-
ther information regarding the optimization process, see the
Supporting Information. Because the use of [PdCl
or Pd(OAc)2 as catalyst proved ineffective (Table 1, entries 2
2ACHTUNGTRENNUNG(PPh3)2]
AHCTUNGTRENNUNG
and 3), we focused on the application of copper bromide. In
its absence, no product formation occurred (Table 1,
entry 4).
To reduce the extent of the homocoupling of 1a,[7a] the
loading of 2a was increased to 30 equivalents, which en-
hanced the yield of 3a significantly to 65% after stirring at
room temperature for 14 h (Table 1, entry 5).
[a] L.-H. Zou, J. Mottweiler, Dr. D. L. Priebbenow, Dr. J. Wang,
J. A. Stubenrauch, Prof. Dr. C. Bolm
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Fax : (+49)241-8092391
The use of bathophenanthroline as an alternative to 1,10-
phenanthroline was also successful, the product yield re-
maining almost the same (Table 1, entry 6). In the absence
E-mail: carsten.bolm@oc.rwth-aachen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204502.
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COMMUNICATION
Table 1. Direct oxidative cross-coupling reaction of 2-phenyl-1,3,4-oxa-
Table 2. Copper-mediated direct arylation of 2-aryl-1,3,4-oxadiazoles
diazole (1a) with pentafluorobenzene (2a).[a]
(1a–i) and benzoxazole (1j) with pentafluorobenzene (2a).[a]
Entry
Metal source
CuBr
Base
Oxidant
Yield [%]
Entry Heterocycle
1
1
Product
3
Yield [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuONa
tBuOK
tBuOLi
O2
O2
O2
O2
O2
O2
O2
34
A
N
0[b]
0[b]
0
1a
3a 65
3b 68
Pd
–
ACHTUNGTRENNUNG(OAc)2
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
65[c]
60[c,d]
24[c,e]
0
2
3
4
1b
1c
1d
PhIACTHNGURETNNU(G OAc)2
Selectfluor
AgOAc
–
O2
O2
O2
0
0
0
16
3c 67
3d 65
3e 61
12
64[c,f]
[a] Reaction conditions: 1a (0.2 mmol), 2a (1.0 mmol), metal source
(0.2 mmol), 1,10-phenanthroline (0.2 mmol), base (0.6 mmol), CH3CN
(1.5 mL), RT, 14 h. A balloon of O2 was used in reactions with dioxygen.
[b] Use of 5 mol% of the metal source. [c] Use of 30 equiv of 2a. [d] Use
of bathophenanthroline instead of 1,10-phenanthroline as ligand. [e] No
ligand. [f] The purity of CuBr was 99.999%.
5
6
1e
1 f
3 f 63
of a ligand, the yield of 3a decreased dramatically (entry 7).
Attempts to synthesize 3a by using other oxidants such as
PhIACHTUNGTRENNUNG(OAc)2, Selectfluor, and AgOAc were unsuccessful (en-
7
8
1g
1h
3g 64
3h 62
tries 8–10). In addition, compound 3a was not formed in the
absence of dioxygen (entry 11). Bases other than tBuOLi
(tBuONa, tBuOK, Na2CO3, and Cs2CO3) were successively
tested in this reaction, although the desired product 3a was
obtained in lower yields or only trace amounts. To exclude
the possibility of contamination by other catalytically active
transition metals, a control experiment was performed by
using CuBr with a purity of 99.999%;[18] the desired product
was obtained in a similarly good yield (entry 14).
With the optimized conditions in hand (see footnote [a]
of Table 2), the scope of the reaction was investigated. A
broad range of substrates with electron-rich or -deficient
aromatic substituents at the 2-position of the 1,3,4-oxadi-
9
1i
1j
3i 45[b]
10
3j 20[c]
[a] Reaction conditions: heterocycle 1 (0.2 mmol), 2a (6 mmol), CuBr
(0.2 mmol), 1,10-phenanthroline (0.2 mmol), tBuOLi (0.6 mmol), O2 (bal-
loon), CH3CN (1.5 mL), RT, 14 h. [b] 1 mmol scale; use of 10 equiv of
2a. [c] 1 mmol scale; use of 5 equiv of 2a.
ACHTUNGTRENNUNGazole were tolerated (Table 2, entries 1–8). By using ten
equivalents of 2a, 2-(3-pyridyl)-1,3,4-oxadiazole (1i) afford-
ed the desired product 3i in 45% yield (1 mmol scale;
entry 9). The coupling of benzoxazole (1j) with pentafluoro-
benzene (2a, 5 equiv) provided the corresponding product
3j in a low yield of 20% (entry 10).
By using only five equivalents of 1-cyano-2,3,5,6-tetrafluoro-
benzene, product 3t was obtained in 50% yield. With elec-
tron-rich 1-methoxy-2,3,5,6-tetrafluorobenzene the yield de-
creased, product 3s being obtained in a yield of only 15%.
With 1,2,4,5-tetrafluorobenzene (10 equiv) as the coupling
partner of 1a, a mixture of 3u and 3v was formed, and the
products were isolated in 8 and 9% yield, respectively. With
three equivalents of 2,3,5,6-tetrachloropyridine, the cross-
coupling reaction with 1a proceeded smoothly, leading to
the formation of 3w in 35% yield. Attempts to use tri- or
difluoroarenes failed, most likely due to the significant dif-
ference in acidity between these fluoroarenes and 1,3,4-oxa-
diazoles.[7b]
The protocol was also applied to a range of tetrahalo-
ACHTUNGTRENNUNG(het)arenes containing additional substituents (Scheme 2).
In these cases, the amount of polyfluoroarene could be re-
duced to ten equivalents (or less). Tetrafluoroarenes bearing
electron-withdrawing substituents efficiently coupled with a
variety of 2-aryl-substituted 1,3,4-oxadiazoles, furnishing the
corresponding products in good yields. For example, 1-tri-
fluoromethyl-2,3,5,6-tetrafluorobenzene reacted with oxa-
diazoles 1a–h to give the products 3k–r in 50–61% yield.
Chem. Eur. J. 2013, 19, 3302 – 3305
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C. Bolm et al.
Scheme 3. Proposed reaction mechanism.
À
AHCTUNGTRENNGaUN zoles with polyhaloarenes by dual Csp2 H activation. It is
noteworthy that the reactions proceed under mild reaction
conditions at room temperature by using an inexpensive
copper source and dioxygen as oxidant. The process pro-
vides a concise access to biaryl structures containing poly-
AHCTUNGTREGhNNNU aloarenes, which are of interest to the fields of pharma-
ceuticals and functional materials. In this particular process,
molecular oxygen and acetonitrile play crucial roles. The de-
velopment of catalytic variants and the application of the
strategy to other arene systems are currently underway.
Scheme 2. Copper-mediated direct arylation of polyhalo
2-substituted 1,3,4-oxadiazoles. Reagents and conditions: 1,3,4-oxadiazole
(0.2 mmol), polyhalo(het)arene (2 mmol), CuBr (0.2 mmol), 1,10-phenan-
ACHTUNGTREN(NGNU het)arenes with
ACHTUNGTRENNUNG
throline (phen, 0.2 mmol), tBuOLi (0.6 mmol), O2 (balloon), CH3CN
(1.5 mL), RT, 14 h. [a] 1 mmol scale. [b] Use of 5 equiv of polyfluoro-
ACHTUNGTRENNUNGarene. [c] Products 3u and 3v were obtained as a mixture and isolated in
8 and 9% yield, respectively. [d] 0.5 mmol scale; use of 3 equiv of 2,3,5,6-
Experimental Section
tetrachloropyridine and 4 equiv of tBuOLi (0.4m).
General procedure for the oxidative cross-coupling—reaction between 2-
phenyl-1,3,4-oxadiazole (1a) and pentafluorobenzene (2a):
A 10 mL
oven-dried Schlenk tube equipped with a magnetic stirring bar was
charged with CuBr (0.2 mmol, 28.8 mg), 1,10-phenanthroline (0.2 mmol,
36 mg), and tBuOLi (0.6 mmol, 48 mg). The tube was then sealed with a
rubber septum and filled with O2 by using standard Schlenk techniques.
After adding CH3CN (1.5 mL) through a syringe, the reaction mixture
was stirred at room temperature (258C) for 5 min. Then, 2a (6 mmol,
664.1 mL) and 1a (0.2 mmol, 29.2 mg) were sequentially added under an
atmosphere of oxygen. An oxygen balloon was then connected to the
Schlenk tube by a needle and the reaction mixture was stirred at room
temperature for 14 h. Then the reaction mixture was diluted with ethyl
acetate. The resulting solution was directly filtered through a pad of
silica gel and concentrated under reduced pressure. Purification by
column chromatography (silica gel, eluent: pentane/ethyl acetate 15:1)
afforded product 3a (0.13 mmol, 40.6 mg) as a white solid in 65% yield.
The formation of crystalline structures with relatively high
melting transitions (3t: 2208C; 3v: 2018C) is another inter-
esting feature of most of the products described here. Fur-
thermore, a dramatic difference between the solubility of 3t
compared with the other products was observed. Although
the electron-withdrawing capability of the cyano group and
the fluorine atoms is similar, compound 3t is poorly soluble
in a number of polar solvents (EtOAc, CDCl3, [D6]DMSO,
and CD3OD), which contrasts with the behavior of the cor-
responding all-fluoro derivatives such as 3a.
On the basis of recent reports[7b] on other copper-cata-
lyzed oxidative cross-coupling reactions and our observa-
tions, we postulate the reaction mechanism depicted in
Scheme 3. As mentioned above, the copper-catalyzed aero-
bic cross-coupling proceeds in competition with the forma-
tion of homocoupling byproducts.[7a,c] The desired cross-cou-
pled products would then be formed by reductive elimina-
Acknowledgements
tion of [Cu
A
ACHTUNGTRENNUNG
This work was supported by the Fonds der Chemischen Industrie.
L.-H.Z. and J.M. thank the Chinese Scholarship Council (CSC) and the
NRW Graduate School BrenaRo, respectively, for predoctoral stipends.
D.L.P. and J.W. are thankful to the Alexander von Humboldt Foundation
for postdoctoral fellowships.
ACHTUNGTRENNUNG
in the formation of the homocoupling products of the poly-
haloarene and the azole. Thus, the use of an excess of poly-
haloarene suppresses the formation of the azole-derived by-
product (as was also observed experimentally).
À
In conclusion, we have developed a copper-mediated
direct oxidative cross-coupling of 2-(het)aryl-1,3,4-oxadi-
Keywords: C H activation
heterocycles · oxygen
· copper · fluoroarenes ·
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Copper-Mediated Oxidative Cross-Coupling
COMMUNICATION
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[17] During the preparation of our manuscript, the following two impor-
tant papers appeared reporting a) the copper-catalyzed dehydrogen-
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AHCTUNGTREGrNNUN oarenes (S. Fan, Z. Chen, X. Zhang, Org. Lett. 2012, 14, 4950) and
b) the dehydrogenative cross-coupling between two azoles (X. Qin,
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77, 7677).
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Received: December 18, 2012
Published online: February 10, 2013
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